Morphologic and biomechanical comparison of spinous processes and ligaments from scoliotic and kyphotic patients

The spinous processes and supraspinous and interspinous ligaments (SSL and ISL, respectively) limit flexion and may relate to spinal curvature. Spinous process angles and mechanical properties of explanted human thoracic posterior SSL/ISL complexes were compared for scoliosis (n=14) vs. kyphosis (n=8) patients. The median thoracic coronal Cobb angle for scoliosis patients was 48°, and sagittal angles for kyphosis patients was 78°. Spinous processes were gripped and four strain steps of 4% were applied and held. Percent relaxation was calculated over each step, equilibrium load data were fit to an exponential equation, and a Kelvin model was fit to the load from all four curves. Failure testing was also performed. Median ligament complex dimensions from scoliosis and kyphosis patients were, respectively: ISL width=16.5 mm and 16.0 mm; SSL width=4.3 mm and 3.8 mm; ISL+SSL area=17.2 mm and 25.7 mm; these differences were not significant. Significant differences did exist in terms of spinous process angle vs. spine axis (47° for scoliosis and 32° for kyphosis) and SSL thickness (2.1 mm for scoliosis and 3.0 mm for kyphosis). Fourth-step median relaxation was 42% for scoliosis and 49% for kyphosis. Median linear region stiffness was 42 N/mm for scoliosis and 51 N/mm for kyphosis. Median failure load was 191 N for scoliotic and 175 N for kyphotic ligaments. Differences in loading, relaxation, viscoelastic and failure parameters were not statistically significant, except for a trend for greater initial rate of relaxation (T1) for scoliosis ligaments. However, we found significant morphological differences related to the spinous processes, which suggests a need for future biomechanical studies related to the musculoskeletal aspects of spinal alignment and posture.